Reinvestigation of thermoelectric properties of n- and p-type Ba _8-d Au _x Si_46-x-y clathrate

We have synthesized n- and p-type clathrates Ba_8-d Au _x Si_46-x-y with various Au contents (4.6 < x < 6.0) by arc-melting, annealing at 1173 K, and spark plasma sintering at 1073 K. The Au compositions found by wavelength-dispersive x-ray spectrometry for the synthesized samples were slightly lower than the nominal compositions. Ba _7.8Au_4.6Si_41.4 and Ba_7.7Au _4.9Si_41.1 samples showed n- and p-type conduction, respectively. According to the electron count (Ba²⁺) ₈Au(^3-)_5.33Si_40.67, the clathrate composition with x = 5.33 is expected to be an intrinsic semiconductor. Our experimental results show that increase of the Au composition causes a transition from n-type to p-type conduction between x = 4.6 and 4.9. We have also calculated the band structures of the Ba₈Au _x Si _46-x clathrate including a vacancy by ab initio calculation based on density functional theory with structure optimization. It was found that the vacancy behaves like an electron acceptor and the numbers of vacancies at 24k sites for the synthesized Ba₈Au _x Si_46-x-y clathrates can be estimated as ∼0.4 in a unit cell.